Synchronous motors or synchronous machines are suitable for the generation of an adjustable torque response. Synchronous machines can be made to generate a constant torque by supplying them with trapezoidal electromotive forces and square wave currents. The square wave currents are generated in a power converter after preceding pulse width modulation. This pulse width modulation is aided by a pulse distribution circuit which is a function of the rotor position wherein a rotating electromagnetic field originates in the machine. Because the synchronous machine is not ideally constructed, errors occur such as gaps in the windings, slot ripple, manufacturing tolerances, etc., and, as a result, the resulting torque response deviates from the nominal torque. Additionally, because of the controlled adjusting of the current and time settings having different lengths in the up and down commutation, system-related torque transients result.
In the German Patent Application, No. P 37 36 215.5, a method is suggested for avoiding a current transient in a phase winding of a three-phase square wave synchronous machine, wherein the three phase square wave current is not part of the commutation process. The pulse width modulated pulses for the power converter of the synchronous machine are generated by a high frequency triangular scanning potential and a control potential. Through an anticipating commutation control, on the average, the current decrease in the down-commutated phase winding is equal to the average of the negative current increase in the up-commutated phase winding. This method leads to a marked improvement in the torque response. The torque transients however can only be reduced in terms of the time average and not instantaneously.
Thus, there is a need for a method that produces a constant torque response in the regulation of electric motors, particularly synchronous machines.